The invention relates to improvements in modular building systems (also called structural frameworks or sectional frames) in general, and more particularly to improvements in modular building systems and in components of modular building systems of the type disclosed, for example, in published European patent application No. 0 010 475, in European Pat. No. 0 136 431, in U.S. Pat. Nos. 3,451,183, 3,513,606 and 3,559,357, and in German Utility Model No. 19 19 678.
Modular building systems of the type referred to in the above-enumerated application, Utility Model and patents normally utilize elongated profiled frame members which are made of light metal and have smooth, substantially plane longitudinally extending external surfaces as well as at least one longitudinally extending core which is provided with at least one longitudinally extending hole. Each frame member further comprises at least one longitudinally extending chamber and walls which surround the chamber and are connected to each other as well as to the core. The frame member which is described and shown in European patent application No. 0 010 475 is provided with two chambers which are disposed at opposite sides of the core and communicate with longitudinally extending grooves in the external surfaces of the frame member. Each chamber is surrounded by a split undercut outer wall having a generally C-shaped cross-sectional outline and by two internal walls which extend radially of the core and separate the chambers from each other. If one wishes to assemble two frame members into a building system wherein the frame members extend at right angles to each other, it is necessary to employ a bolt having a head which is sufficiently small to pass through the groove in one of the outer walls of one frame member. The shank of the bolt is caused to pass through a transverse hole of the core in the one frame member, through the groove in the other outer wall and axially into the hole of the core of the other frame member. This establishes a connection which maintains one end face of the other frame member in abutment with an external surface of the one frame member. The head of the bolt bears against the core of the one frame member and can be reached by the working end of a suitable tool which is used to rotate the bolt, either in a direction to drive the shank axially into the core of the other frame member or to disengage the shank from the other frame member. A drawback of the frame members which are disclosed in European patent application No. 0 010 is that their resistance to deforming stresses is rather limited, particularly the resistance of those portions of the outer walls which are adjacent the respective longitudinally extending grooves. Such deforming stresses arise in response to the push or pull of the bolt when the latter is driven home with a substantial force. A pronounced pull entails deformation of the core and of the internal walls, and a pronounced push entails deformation of the outer walls. In addition, frame members of the just outlined character exhibit a rather limited resistance to torsional stresses.
German Utility Model No. 19 19 678 discloses a frame member which has a square cross-sectional outline with a centrally located elongated core having a longitudinally extending hole, with four outer walls each of which has a centrally located longitudinally extending groove, with four longitudinally extending chambers which surround the core and each of which communicates with one of the grooves, and with four diagonally extending internal walls which separate the neighboring chambers from each other and each of which extends from a corner of the core to a corner between two neighboring outer walls. Each chamber has a trapeziform cross-sectional outline, and the halves of two outer walls which are connected to the outermost portion of a diagonal internal wall resemble an arrowhead. If a first frame member is to be separably connected with a second frame member in such a way that one end face of the second frame member abuts an external surface of the first frame member, one employs a threaded fastener in the form of a bolt or screw with a head which is slid into one chamber of the first frame member. The shank of the fastener extends outwardly through the respective groove add axially into the hole in the core of the second frame member. The head of the fastener bears against the internal surface of the corresponding outer wall at both sides of the respective groove and is likely to deform the two halves of such outer wall in response to the application of a force which pulls the end face of the second frame member away from the adjacent external surface of the first frame member. Furthermore, the diagonal walls between the chambers are incapable of standing pronounced flexing stresses, the same as the core, so that such frame members exhibit a very low resistance to compressive as well as torsional stresses. For example, a relatively small force which tends to move one grooved outer wall toward a grooved outer wall opposite the one outer wall entails pronounced flattening of the frame member with attendant widening of grooves in the one and the other outer wall and with equally pronounced narrowing of the grooves or gaps in the other two outer walls. The ability of the just described frame members to withstand torsional, compressive and/or other stresses does not improve if they are provided with several cores and/or with differently oriented internal walls between neighboring chambers.